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by Daniil BERMAN, Cand. Sc. (BioL), head of the Biocenology Laboratory, Institute of Biological Problems of the North, RAS Far-Eastern Branch (Magadan)
The territories of Atlantis, Arctis and Beringia - now an ocean floor-share a common destiny. But unlike the legendary Atlantis and Arctis, Beringia is a hard reality, though not devoid of mythological either. We know that 120 to 140 centuries ago the level of the World Ocean was 90 meters lower than today. The shallow Bering Strait would go dry now and then become a land "bridge" linking Asia and America. That passage, incidentally, was wider than Alaska, let alone Chukotka, the Chukot Peninsula. The sea shelf, laid bare over hundreds and hundreds of kilometers, contributed to the accretion of land, especially along Siberia's north and east, and at the expense of the Chuckchee and Bering Seas (as to the Sea of Okhotsk, nearly all of it went dry). That continental shelf, merging with Chukotka and Alaska, gave rise to a giant land united geographically, Beringia. Clear of ice by and large, Alaska and Canadian Yukon were connected with inland territories of the continent only by a narrow strip ("Beringia's tail", as D. Hatry has aptly put it), squeezed in between two huge mountain glaciers-the St. Lawrence and the Cordilleran shields. The mountains of Chukotka and of the Indigirka and Kolyma rivers basins were likewise under glaciers which, however, could stand no comparison with the American ones.
Those parts had a rich and varied animal kingdom which abounded in herbivores (mammoths, bisons, northern deer, marmots, gophers, lemmings) and in carnivores (bears, wolves, gluttons, polar foxes) as well. Buried in the tundra are their bones - whole cemeteries in fact - brought to the ground surface by rivers and streams, and sometimes just sticking out from the soil.
It goes without saying it is a formidable job of work trying to reconstruct the now extinct landscapes. The world does not know of examples like that. A mere listing of flora and fauna ecologies will take us nowhere and give birth to chimeras of every kind. That is why all the various specialists - geologists, paleontologists, climatologies and those involved with permafrost - often voice different opinions when it comes to characterize at least some of the basic features of tundra-plain landscapes of Beringia.
At first sight it looks quite simple where Beringia's landscapes are concerned. Why not proceed from what we know about the mammoth, the best-known beast of the glacial periods? But as things actually stand, we know all too little about that animal, even certain essential things about it. For instance, what kind of life did it lead? A settled mode like the present musk ox, or a nomadic one like the northern deer-
an animal roaming in the tundra plains in the summertime, and moving south, to the forest-tundra, in winter? Did the mammoth feed on tree and shrub twigs like do the elephants of today? Or was it perhaps something else - grass or lichens- which it might have been digging with the lower convolutions of its tusks? How did it get water in wintertime? By snatching snow as many arctic animals still do? And finally, what were its surroundings - regular tundras, arctic prairies or hypothetical plains, what we might call tundra steppes? As a matter of fact, mammoths themselves argue in favor of... lush meadows (arctic prairies). Now, this is a simple line of reasoning: no trees, if any, which means they, the mammoths, must have subsisted either on grass or on lichens (their teeth were adapted to that). If it was grass, it must have been in abundance (there could not be as much lichen as that). Hence the conclusion: mammoths inhabited high-grass landscapes, to use our specialist lingo - or rather, arctic prairies, if you want a romantic twist to it... As you see, we know but precious little about the mammoths. The holdovers of their breed, however, could still be found on the Vrangel Island a little over thirty centuries ago, that is when the Egyptians could already read and write.
INSECTS AS ENVIRONMENT INDICATORS
Unlike the pollen of plants and the bones of animals, which wind or water can carry away over great distances without any substantial damage - chitin, the horny substance forming the hard outer covering in insects, is more vulnerable and crumbles readily under the action of water, sand or pebble. Therefore the chitin of insects, if found in deposits, is proof enough that insects inhabited the site ages ago. Many insects, in fact, are rather squeamish about their habitat, a quality that makes them excellent indicators of the ambient environment, its temperature and humidity characteristics, among other things. Furthermore, in contrast to plant pollen and spores, chitin allows us to determine an insect species with very good reliability.
These factors, we hope, may help us answer three intriguing questions to begin with. First, what were the tundra steppes like? Were they like high-grass meadows capable of giving food to a multitude of huge herbivorous animals? Or like the tundra plains of today with small dry patches offering but sparse blades of grass? That's the first point. The second question: what was the climate like in Beringia during cold epochs? The mean temperature in January is believed to be down to -60 0 on the centigrade scale and even less, with summers rather cool too. And the last point: was Beringia a solid land geographically? And how did the northeast of Asia and the northwest of American differ from each other? These regions are thought to have been tundra plains - or rather steppes, while the dry Bering Strait must have been a moist and cool filter of sorts in the way of xerophilous biota (plants and animals capable of thriving in a hot, dry climate).
Entomologists (experts involved with the study of insects) working in northern latitudes know it quite well that the chitin of fossil insects found in the soil deposits of tundra plains (from the Lena to the Anadyr rivers in the east) belongs to 200-220 species; a significant part of these remains (60 to 80 percent) comes from the beetle Morychus viridis, of the family of pill beetles. It is a small bug with metallic shine, about half as big as the common ladybug, though not round but somewhat elongated, just like a regular pill; hence the name of the family, the pill beetles. (The deposits, by the way, date back to the cold glacial epochs of the Quaternary period.) The chitin of Morychus viridis is in no way superior in durability - a regular substance, neither harder, thicker nor more elastic than in other beetles. This means that the beetle used to be quite prolific there. Its remains are detected throughout the northeastern part of the Russian Arctic, and in deposits of different ages at that. So this little bug could live anywhere.
While studying the present population of beetles in the upper reaches of the Kolyma (and trying to set as many primitive box traps as possible), we hit upon the Morychus viridis - well still alive and crawling! Yes, indeed - in some places its population reached forty bettles per square meter. What's more, this beetle species was found to occur throughout northeastern Asia, though always within the same biotype, that is in highland cold steppes, and coexisting with the community of the smallish xerophilous Carex argunensis and amongst scantlings ofsteppeland and, occasionally, tundra plants, but always in the presence of the moss Polytrichum piliferum. It is on this little moss (frail and stunted as it is because of the rigors of the north) that the larvae of Morychus live and feed. No other plants are good for this cute beetle.
That is why Morychus is a welcome aid to us for identification of landscapes and, what is most important to us, it helps reconstruct its natural environment centuries ago. Today such biotypes are quite rare, they are found in districts
characterized by all kinds of extreme conditions, be it temperature, humidity or wind. But how was it in the Pleistocene? Here we have to admit - judging by the long time of the process of deposition with the remains of Morychus within-that the dominant landscapes were dry, windy and without snow. And the most important point - the productivity of those climes was all too low.
Now, how could we correlate the indisputable existence of numerous herbivores with the possible scarcity of the plant cover? Could this ever be? Yes, it could. Let's recall that the musk ox on the Vrangel island, with its actually snowless winters, lives within biotypes known for poor vegetation. Should we relate, after all, herbivorous animals to predominant landscapes? For instance, the elk in a larch-tree forest will die of famine, but they will fare quite well along rivers flowing amidst the same larch forests, because river valleys are rich in willows, aspens and young pine-trees.
Now doubt, there will be quite a few opponents of our "Morychus" model among the scientific community because, it is argued, a large mammoth could find enough food and survive only on high-grass pastures. But we might just as well recall that no one has ever tried to make a true assessment of the actual population of mammoths. Here's a real example. All paleontologists must have heard about the site called Duvanny Yar on the Kolyma and dated to the Quaternary. It stretches for dozens of kilometers along the high steep bank of this river with numerous bone remains in the deposits. In August 1997, as the water subsided to lay bare a strip of the bank under a precipice, one of our paleoecologists, S. Zimov, found the remains of more than ten mammoths - scattered just over a stretch of two kilometers!
The count system is quite simple: five hipbones - that's two mammoths; two left tusks - another two mammoths; a big tusk and two smaller ones on the right side - yet another three mammoths. And so on down the line. How many mammoths could be buried there per square kilometer? A terrific count! We can only guess how much time these bones must have lain over there, under the precipitous bank of the Kolyma... But don't forget these bones were washed away from deposits that had taken 40 thousand years to form. Some of the dead animals, mind you, were devoured by predators or else carried off by the river downstream, to huge estuaries, to pile up there. We cannot tell how many meters the water has receded to add dry land to the bank. Nor can we tell what has caused intensive sedimentation to bury fast the bone remains. All kinds of things could have happened anyway. What we can tell is this: the vast piling of bones is the result of their conservation in the absence of air and at steadily low temperatures, something that conserves even rocks otherwise destroyed by water freezing and defreezing. Water in the cracks or pores of the bones would have done
the same thing much faster. So, there might never have been huge herds of mammoths that needed rich pastures for food.
The problem of Beringia's climate is even more complicated than that of vegetation.
Let us take our Morichus beetle again.
Indifferent to temperatures, this insect enables us to judge with good confidence about dry soil in summer and strong winds in winter. To understand what Beringia's climate was really like - when both regular steppe and tundra species could coexist within the same biota-let's turn to other beetles, the denizens of those parts. But first, a few words about steppes and tundra plains in Asia's northeast.
Over there, small patches (from 2.5 to 12.5 acres) of steppeland vegetation have survived, populated by associated steppeland insects. Such sites are found along the valleys of large rivers. The very existence of exotic communities amidst the northern larch-tree taiga - what might look like a hostile environment to them-has become possible for a variety of factors. First, these plots lie on steep valley slopes facing south; second, in this highly continental climate even meager sunshine (compared with the southern latitudes, of course) is not dissipated in a lucid, dust-free atmosphere, but used to the full for warming the topsoil which heats to 60 0 C in the summer season; a sum total of temperatures during the warm season attains to 2,500 0 C, that is as much as in the mountain steppes of Altai or Tuva. Permafrost is still there, true, but it lies 2.5-3 meters deep; it is not in solid chunks of ice but in dry crystals, and thus does not affect the topsoil. High temperatures dry up topsoil - the evaporating power rate in July is about 200 mm, or like in semideserts in the lower reaches of the Volga. Some beetle species thrive in this warm and dry microclimate, in particular, the thermophilic weevils (Curculionidae) and leaf beetles (Chrysomelidae) - insects that are of special interest to us.
Since these steppeland isles are totally isolated by dense taiga forests and bogs, impassable for steppeland creatures, they can be considered undeniable relicts of the cold epochs of the Pleistocene. This looks like the best model for reconstructing Pleistocene communities. However, Morychus occurs but rarely on relict steppe patches, while quite nearby, just a few kilometers away and often on the same level (on wind-blown sites though), this species is quite common - or else, it may be a population of regular beetles. Yet here we do not encounter our dear thermophiles, the heat-loving weevils and leaf beetles. Thus if our model proceeds from steppeland weevils and leaf beetles, it will take us to high soil temperatures and "hot" steppes, similar to those of southern Siberia and Mongolia. And if we proceed from Morychus, we'll have little snow and strong winds in winter, and insignificant productivity of plants. Both models have one thing in common - dryness.
Barring very high latitudes, tundras are a maritime phenomenon by and large, caused the proximity of cold polar waters, ice-free, be it only for very short summers. It's chilly, it's dank over there! But what about the dry, sun-drenched steppes? A figment of imagination? If so, steppeland species could never be there. But they were after all!
Let's try to make an estimate of the needs of steppe and tundra beetles for warmth. And let's do it with the use of hard statistics. We can suggest an interesting method, its gist is as follows. Suppose we have recovered so many insects from deposits. We wish to know at what temperatures (say, mean temperatures for July and January) they lived. This is a technically simple problem: we take climate logbooks and write out the values of July and January temperatures recorded by meteorological stations closest to the habitats of present-day insect species. From these data we can plot a "temperature area", or range of existence, for every species. Now let us superpose all these ranges. The more species we have, the smaller the region of concurring values (ideally, it could be a point). This region describes July temperatures on one axis, and the temperatures of January - on the other; both would be acceptable to all the species under study This very region could give an insight into the climate of the deposition time.
For steppe species the mean values of temperature ranges for July proved equal to 17-18 0 C (such temperatures are characteristic of Tuva's central steppelands); for tundra species, the mean July temperatures stood at a mere 7 to 9 0 C (that's like on the Taimyr Peninsula in the Arctic North). We are speaking of the mean July temperatures. Still, there is a common range of July temperatures equally good for the life of steppe and tundra species alike. This is a narrow range, from 11 to 14 0 C, perhaps even less. Even today, at Chersky in the lower reaches of the Kolyma, the mean July temperature is at 12.1 0 С. However, the community of Chersky is situated in the forest zone, while the border of the tundra zone is a bit farther to the north, in the locality bearing the descriptive name of Krai Lesa ("forest edge"). The outskirts of that community (on the Kolyma River bank just near Krai Lesa) have retained patches of relict steppeland with the associated fauna, albeit sparse. These patches lie only on steep slopes (because lowland tracts are too cool) with a much warmer microclimate.
Such microclimate, characteristic of the habitats of steppeland insects, is in stark contrast with the climate of the ambient regions. In the Pleistocene, tundra plains occupied a large area of the present Indigirka-Kolyma lowland. We might stretch a point and admit that the Pleistocene plains were as warmer as the present steppelands are compared with the surrounding territories. We know that in the Late Pleistocene the seashore lay much farther to the north-enough to make the climate more continental. If we allow for the existence of frozen ground in lowlands in the form of a grid of what we call polygons (water pools surrounded by
levees of swelling frozen ground), our picture will fit in snugly. Spots of steppeland communities with heat-loving beetles will then find their niche on the southern slopes of levees, while sites of cold steppes populated by the Morychus beetle will feel fine on flat "watershed" surfaces of the selfsame levees - rising from under the snow and thus wind-blown in winter. "Beds" of tundra plants with beetles will settle on the northern slopes; as to the water pools - they will produce lush marsh plants to give food to so many herbivores...
Now what concerns the wintertime characteristics. Minimum temperatures of topsoil on the hibernation sites of tundra and steppe beetles are close to modem values: -16 to 18 0 C; what with the mean temperatures for January being -45 to 47 0 C and a low snow cover (because the continental climate means scant precipitation), insects could find comfortable conditions only under snowdrifts. Frozen ground relief is responsible for unevenness of the snow cover, even given small precipitation in winter...
This is a convenient model, but... it lacks factual evidence. Another approach would be more realistic, likewise making it possible to find a proper compromise. Any river in any plain causes rough relief by destroying banks in one place and depositing the washed debris in another to give rise to levees and flood-plain terraces. Their elevated parts dry up better to give refuge to xerophilous plants and invertebrates, while damp spots attract hydrophiles, or moisture-loving species. And wetlands are a welcome medium for marsh organisms.
Now to summarize. Our study of beetles has enabled us to get amazingly high temperatures both for summer and for winter. Much higher than in previous studies. The most surprising thing of all is that the summer temperatures of the Recent Pleistocene differ but little from the present ones - by a few degrees only. So even a rather small climatic shift would be enough to bring about dramatic changes in the landscapes of the North.
Thus far we have not succeeded in wedding our three conflicting models relying on the Morychus, thermophilic beetles and tundra beetles, respectively. But this is by no means a routine task.
FROM CHUKOTKA TO ALASKA
Botanists involved with the contemporary flora and paleontologists studying the mammoth-populated fauna agree that Alaska, free of ice in the Pleistocene, and Canadian Yukon were an extension of northeast Asia. The American paleontologist C. Repenning way back in 1967 wrote about the detected fossil remains of field-voles of the Siberian fauna dating from the cold spells of the Alaskan Pleistocene. And the American paleoentomologists D. Matthews and A. Telka called Yukon a blind alley of the mammoth-inhabited Asian plains. A giant of Asian Beringia, the woolly rhinoceros, is not known in America. That animal was common in Yakutia's northeast and in the Kolyma River basin,
where its frontal (large) horns were found in an excellent state of preservation. In other words, Asian animals could cross into America, while Asia happened to be "off limits" to many American natives. A vivid instance of that is seen in the American short-snout bear, combining the grace of a cheetah (and just as quick) with the dullness of a hyena's muzzle. Not found in Asia either are the American camel, the ground sloth, the mastodon, the jaguar, the coyote, among others.
So, the animal kingdom of the northern extremities of the two continents is far from being identical. Hence the notion about the dank "filter" in Beringia's central part (broadly, the territory of the Bering Strait that went dry) meant to explain this paradox. That region used to be less continental at the time than were Chukotka and Alaska.
Judging by the makeup of species common to both continents and proceeding from climatic models, we can suppose that the climate of central Beringia corresponded to the conditions of present zonal tundras in air temperature and humidity, and was even "worse" according to other characteristics. This conclusion is fully consistent with the data obtained from the study of the remains of plants and insects recovered from the floor of the Bering Strait.
And here's evidence that the dank "filter" of the once dry Bering Strait worked indeed: each group of organisms, regardless of its size, was found to have but few common species of the steppes - just above a dozen. More than that, the number of true thermophiles (heat-loving denizens of the steppes) is even less: only one species of bugs, butterflies and spiders, respectively, whose taxonomy, ecology and propagation have not been studied well enough. The other steppe species, the xerophiles living in a dry climate, do not depend overmuch on summer warmth and can occur also in forest and even in tundra zones. In short, we cannot regard them as indicators of a medium along migration pathways. These species are not the true signs of steppes on passage ways, for they could have migrated much earlier, in the Late Pliocene and in the Early Pleistocene rather than in the Late Pleistocene; they could have done it through forest glades because of the general cooling in Trans-Beringia's forest belt-the forests turned ever drier and lighter due to the substitution of light conifers and close- leaved trees for dark, shade varieties. Judging by the population of common species of insects and spiders, the Bering bridge could never be a steppe or tundra-steppe in the Pleistocene.
This important, statistically substantiated conclusion (based on the study of hundreds and hundreds of invertebrates) is also confirmed by paleontological data. The Pleistocene deposits of America have no beetles of the Asian steppes so much typical of northeastern Asia's lowlands during the Pleistocene. They did not penetrate North America, and so the beetle population of northwestern America is composed of native species for the most part. That is to say, the dank "filter" of the Bering isthmus let in mammalians and steppeland plants, but it did not steppe beetles, more sensitive to environmental conditions. We know of situations like that. For instance, among the tundra-plain communities of the Vrangel Island we have found no invertebrates of the steppe species. And for all the varied steppe flora on Chukotka, we encounter but single representatives of invertebrate species of the steppes.
All that looks quite plausible. The trouble is that not only steppeland species did not make use of the Bering bridge-many other species did not cross it either-by no means thermophiles, zerophiles and steppe creatures. Take the northern bumblebees, those lovers of dank air. They occur in 20 species in northeastern Asia and in as many in Alaska and Yukon; however these regions share only four species in common (found in the Arctic and in the subarctic mountain tundras). Yet another dozen bumblebee species are found in the subarctic highland of northeast Asia - they do not hail from steppes either. Even on the Vrangel Island we can find bumblebee species that have never reached Alaska. I can give a lot of other examples to this effect. Here's one, the riddle of the commonplace earthworms. In the Asian northeast we have found two species: one inhabiting the Sea of Okhotsk coast only, the other, eisenia (Eisenia nodenskioldi) , everywhere. For this very species, eisenia, the northeast is the fringe of a vast area taking in the entire region of permafrost and a very wide belt to the south. The earthworm occurs in Europe's east, too, even far in the south. The worm braves hard frosts down to -40 0 C, and can winter at any stage of growth. In summer it can make do with the meager warmth of tundra soil and basks in the sun, something quite deadly to most of the soil fauna; it can do without food for months on end waiting for rainfall in hermetic capsules it builds. The eisenia worms can bear many other bad things. That is why its propagation area is so great, from humid spots in steppes in the south to northern tundras, including the isles of the Sea of Okhotsk and the Vrangel Island. Yet this worm species is not found in America, neither is it in Alaska nor in Yukon. This phenomenon defies explanation.
There is yet another hypothesis, parallel with that of the Bering "filter". The native communities of Beringia, well adapted to the harsh conditions of life there and thus more competitive, did not allow in American visitors. Is it because of competition that the woolly rhinoceros, Asian bumblebees, locusts (Locustidae), eisenia earthworms and other species do not occur on the American shore of the Bering Strait? Or is it the "filter" again? Or perhaps their cumulative effect? We cannot tell. But one thing is obvious: Alaska and Yukon were a landscape extension of Asian Beringia and its tundra steppes-for mammals and to some extent for plants as well. No doubt, Asia's northeast and America's northwest made little, if any, difference to bisons or mammoths; and so did European south, where both thrived in the Pleistocene. But these territories were a world of difference to invertebrates.
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